Harry Kurreck

Electron donor–acceptor compounds: exploiting the triptycene geometry for the synthesis of porphyrin quinone diads, triads, and a tetrad

Abstract Rigidly and covalently linked porphyrin quinones are well established as model compounds for studying photo-induced electron transfer (PET) reactions like those occurring during the primary processes of photosynthesis. In this context, the synthesis of a number of porphyrin quinones is reported in which one or two porphyrin electron donors are connected to either one or two quinone electron acceptors, resulting in diad, triad, and tetrad model systems, respectively. The porphyrin(s) and the quinone(s) are linked by triptycene, 1,4-phenylene, and cis - or trans -1,4-cyclohexylene bridges. The use of the 1,4-phenylene, and cis - or trans -1,4-cyclohexylene bridges results in donor–acceptor compounds with the same number of bonds between donor and acceptor(s), but differing in distance and orientation. Analysis of the 1 H NMR spectra confirmed the chair conformation for the cis - and trans -cyclohexylene-linked diads and triads. NOE experiments gave information about the spatial arrangement of the target compounds. The key compounds in the syntheses of all these new PET model systems are the triptycene quinones, which are formed via a [π s 4 +π s 2 ]-cycloaddition between an anthracene derivative and a suitable quinone. The triptycene system enforces a rigid orientation on the quinone acceptor(s) in the final model system. Evidence is given that the triptycene system has further potential for constructing tailor-made donor–acceptor compounds.

Photo‐induced electron transfer in covalently linked donor–acceptor assemblies in liquid crystals. Time‐resolved electron paramagnetic resonance

Intramolecular electron transfer (ET) in cis and trans isomers of a covalently linked porphyrin–cyclohexylene–quinone, oriented in liquid crystals (LCs), is monitored by time‐domain electron paramagnetic resonance (EPR) spectroscopy over an extended range of temperature, i.e., 210<T<320 K. The spectra of both isomers exhibit different line shape behavior and temperature dependence, as compared to those found in isotropic solutions (ethanol). Whereas in ethanol the range of detection is quite narrow, i.e., 130<T<160 K, the one in the LC environment includes also the nematic fluid phase, thus allowing one to monitor the ET products in this phase. The difference in spectra between the two environments is analyzed in terms of the magnetization projection on the LCs director, L. The different spectra of the two isomers are interpreted in terms of their different molecular geometry. In the case of the trans isomer, both triplet‐ and probably singlet‐initiated ET routes can concurrently be detected, and the free...

Structure and Conformation of Photosynthetic Pigments and Related Compounds. 12. A Crystallographic Analysis of Porphyrin-quinones and Their Precursors

Abstract. A comprehensive crystallographic analysis of 10 porphyrin quinone precursors (dimethoxybenzene derivatives), and six porphyrin quinones has been performed. The free bases and zinc(II) complexes of the porphyrin quinones are of the 5,10,15‐triaryl/alkyl‐20‐quinone‐porphyrin type and carry various bridging and quinone units. The structural and conformational parameters were determined for all compounds; the donor‐acceptor separation distances range from 6.3 to 10.9 Å. Knowledge of these data is a prerequisite for a detailed interpretation of theoretical and spectroscopic studies on such systems. Despite the obvious influence of the type and geometry of the bridging unit and quinone on the spatial arrangement of the donor and acceptor components, a large variety of different packing arrangements in the crystal were observed. These include π stacking, aggregate formation and axial ligation in the zinc(II) porphyrins. The latter often utilized the quinone (or dimethoxy) oxygen atoms for coordination to zinc(II) centers leading to porphyrin quinone dimers and even polymers.

Über galvinole und galvinoxyl-mehrspinsysteme—I : Eine neue metallorganische synthese von mono- und oligo-galvinolen☆

Zusammenfassung Mit Hilfe der neuen metallorganischen Verbindung (2,6 - Di - tert. - butyl - 4 - lithiumphenoxy) - trimethylsilan wird die universelle Einfuhrung des 3,5 - Di - tert. - butyl - 4 - hydroxyphenyl - Bausteins ermoglicht. So lassen sich mit den Estern aromatischer Mono-, Di und Tricarbonsauren die entsprechenden Mono-, Bis- und Tris-galvinole gewinnen. Aus Nitrilen bilden sich korrespondierende Ketone, mittels Dimethylformamid gelangt man zum 3,5 - Di - tert. - butyl - 4 - hydroxy - benzaldehyd. Die praparativen Bedingungen und die Konstitutionssicherung fur die neuen Verbindungen werden beschrieben.

Time-resolved EPR studies of photoinduced electron transfer reactions in photosynthetic model porphyrin quinone triads

Time-resolved electron paramagnetic resonance (EPR) investigations, performed at microwave frequencies of 9.5 GHz (X band) and 95 GHz (W band), on porphyrin quinone quinone and porphyrin porphyrin quinone triads are presented. Time-resolved EPR at different microwave frequencies makes it possible to elucidate the electron transfer (ET) reactions after pulsed laser excitation and to determine their intermediates, the charge-separated radical pair (RP) states. After photoexcitation in the nematic and soft glass phase of a liquid crystal (LC), spin-polarized EPR spectra are observed for the RPs and for the triplet states of the porphyrins created by spin-selective intersystem crossing (ISC) from the excited porphyrin singlet state. The EPR polarization patterns of the RPs are discussed in terms of the favoured decay channels of the photoexcited singlet state of the porphyrin donor. The decay pathway can be singlet ET to the quinone(s) followed by singlet/triplet mixing to yield RPs with triplet character, or...

NOVEL BIOMIMETIC MODELS FOR PHOTOSYNTHESIS : PORPHYRINS COVALENTLY LINKED TO REDOX-ACTIVE CROWN ETHER QUINONES

Abstract The synthesis of biomimetic photosynthetic model compounds, composed of 5,10,15-triphenylporphyrins covalently linked to redox active crown ether quinones via different bridges, is described. Using the zinc complex with the flexible butylene spacer in non polar solvents backfolding of the crown ether quinone over the porphyrin plane is observed by 1H NMR chemical shift studies. This conformational effect is supported by molecular modelling calculations. The backfolding is induced by complexation of the acceptor with the central atom since it is not observed in the metal free-base porphyrin derivative. Due to steric constraints the zinc porphyrin cyclohexylene crown ether quinones lack this backfolding. Cyclic voltammetry measurements show a significant shift of the reduction potential of the quinone crown ether moiety in the presence of alkali metal cations.

Esr, 1H-, D-, 13C-endor and triple resonance studies of alkyl substituted phenalenyl radicals

Abstract A variety of alkyl substituted phenalenes, including deuterated and 13C labeled compounds, was synthesized via an organometallic reaction. From the respective paramagnetic oxidation products, the phenalenyls, ESR spectra and proton, deuterium and 13C ENDOR resonances could be obtained. From the electron-nuclear-nuclear-TRIPLE resonance experiments the signs of all hyperfine coupling constants could be determined. An interpretation of the spin density distributions on the basis of the hyperconjugation model is given. The results of HMO-McLachlan andINDO calculations are reported.

Untersuchung der π-σ-delokalisation an den radikalionen des rubrens mit endor in lösung

Zusammenfassung Von den negativen und positiven Radikalionen des Rubrens (Tetraphenyl tetracen) wurden ENDOR-Spektren in Losung aufgenommen. Die eindeutige Zuordnung der Protonen -hfs-Kopplungskonstanten wurde durch Vergleich mit den ENDOR-Spektren verschiedener teildeuterierter Rubrene ermoglicht. Das positive Vorzeichen einer Phenylkopplungskonstante wurde mit NMR nachgewiesen. Die gemessene Reihenfolge der Phenylkopplungskonstanten |aHmeta|⪢ |aHpara| = |aHortho| widerspricht den Voraussagen von reinen π-MO-Theorien. Die Ergebnisse lassen sich mit einer direkten Delokalisation des ungepaarten π-Elektrons in das σ-System der Phenyl- substituenten deuten. Die Synthese der teildeuterierten Verbindungen wird beschrieben.

ENDOR studies of π-electron delocalization in covalently linked porphyrin dimers. Model systems for the primary donor in photosynthesis?

The spin density distributions in the cation radicals of various covalently linked tetraphenyl porphyrin dimers and of their monomeric constituents have been studied by liquid-phase EPR, ENDOR and TRIPLE resonance methods to find out whether the dimer systems show intramolecular electron delocalization. Such delocalization is known to occur in the bacteriochlorophyll dimer (‘special pair’) in photosynthetic reaction centres. The dimers studied in this work are derived from zinc meso-tetraphenylporphyrin (ZnTPP) moieties linked either by a phenyl ring at para and meta positions or by a —CH2— bridge at para positions. 1H and 14N hyperfine coupling constants were measured for the dimer and ZnTPP cation radicals. By comparing the hyperfine data it was concluded that in the phenyl-bridged porphyrin dimers the unpaired electron is delocalized over the dimer halves (similar to the ‘special pair’) whereas in the CH2-bridged dimer the unpaired electron is localized on one porphyrin unit.

1H and 13C endor on phenoxyl biradicals randomly oriented in frozen solution

Abstract ENDOR measurements have been performed on 13 C labelled biradicals, namely Yangs biradical and p -phenylenebisgalvinoxyl, in frozen solutions. Besides intense ENDOR lines at the free nuclear Zeeman frequencies, hyperfine shifted 1 H and 13 ENDOR lines could be detected with excellent signal-to-noise ratios. From these spectra not only the magnitudes, but also the signs of the hyperfine splitting components relative to the zero field splitting parameter D could be derived. It is demonstrated that the sign of D in Yangs biradical must be negative. This may be due to a spin-orbit coupling contribution to the zero field splitting parameter.